1. Field of Invention
The invention relates to a sheet accommodating device for accommodating a stack of sheets in an image forming apparatus.
2. Description of Related Art
Japanese Utility Model Publication JP-U-62-181530 discloses a sheet cassette that is to be mounted in a printer. A structure of this sheet cassette is shown in FIG. 10. The sheet cassette 101 includes a sheet pressing plate 102 where a stack of sheets are placed thereof, a spring 104 for urging a front end of the sheet pressing plate 102 against a sheet feed roller 103, and a support member 105 for supporting the sheet pressing plate 102 at the center of gravity of the sheets to be stacked so that the sheet pressing plate 102 is swingable vertically. As the front end of the sheet pressing plate 102 is swung upward on the support member 105, a rear end of the sheet pressing plate 102 is swung downward, like a seesaw. The sheet pressing plate 102 is provided with an end guide 106 for supporting rear edges of the sheets to be stacked by moving back and forth.
However, in the seesaw type sheet cassette shown in FIG. 10, the center of gravity of the sheet deviates from the position where the support member 105 supports the sheet pressing plate 102 because the centers of gravity of the sheets vary with the size of the sheets to be stacked. As a result, the sheet pressing plate 102 is not swung with stability.
Further, there is a large difference (Dh) between a height from a bottom of the sheet cassette to the rear end of the sheet pressing plate 102 when few sheets are stacked (h1) and the height when a large number of sheets are stacked (h2). Therefore, there is a problem that the printer becomes large in size.
In particular, in order for the sheet cassette to accommodate a large amount of sheets, the difference Dh becomes larger, so that the printer becomes larger in size. Consequently, it is difficult to make the sheet cassette small in size.
The seesaw type sheet cassette can minimize variations of a pressing force from the spring 104 traceable to a weight change that occurs due to variations in size of the sheets. However, the amount of compression of the spring 104 varies in accordance with the amount of stacked sheets, so that the pressing force from the spring 104 varies and sheet feeding operation becomes unstable.
According to the invention, a sheet accommodating device is provided which can always perform sheet feeding with stability in accordance with the size of sheets or the amount of sheets to be stacked.
In the invention, the sheet accommodating device includes a stacking portion holding a sheet thereon, a first urging member urging one end of the stacking portion upward, and a support member that is movable back and forth relative to the stacking portion and supports the stacking portion near a center of gravity of the sheet to be stacked on the stacking portion.
According to this structure, the support member moves back and forth relative to the stacking portion so as to support the stacking portion near the center of gravity of the sheet to be stacked. Therefore, the support member can be moved back and forth even when the size of the sheets to be stacked on the stacking portion is changed, and thus the support member can support the stacking portion near the center of gravity of the sheet at all times and the stacking portion is swung with stability.
The sheet accommodating device further includes a rear edge support member that supports a rear edge of the sheet and is disposed at a rear end of the stacking portion so as to be movable in accordance with the size of the sheet, and a link mechanism that moves the support member back and forth in accordance with a movement of the rear edge support member.
According to this structure, when the rear edge support member is moved back and forth in accordance with the size of the sheet to be stacked, the link mechanism moves the support member back and forth relative to the stacking member so that the support member supports the stacking member near the center of gravity of the sheets, in synchronization with the movement of the rear edge support member. Therefore, the stacking portion can be supported by the support member at a position near the center of gravity at all times by a simple operation such as moving the rear edge support member in accordance with the sheet size.
The link mechanism acts so that the amount of travel of the support member becomes half distance of the rear edge support member. The link mechanism includes a pinion gear and a rack and the amount of travel is determined by arranging the number of teeth of the pinion gear and the rack. With such a link mechanism, the support member can support the stacking portion at the center of gravity of the sheet.
Further, a second urging member that urges the stacking portion upward may be provided near the support member. When a large number sheets are stacked on the stacking portion, the weight of the stock of the sheets overcomes the urging force from the spring and thus the sheet pressing plate 53 is moved downward. In accordance with the downward movement, the other end of the stacking portion is also moved downward.
On the other hand, when few sheets are stacked, the urging force from the spring overcomes the weight of the stack of sheets and thus the sheet pressing plate is moved upward. In accordance with the upward movement, the other end of the stacking portion is moved also upward.
Therefore, variations in the position of the other end of the stacking portion between a case when a large number sheets are stacked and a case when few sheets are stacked become small. Accordingly, the sheet accommodating device does not need to be large in size and thus it can be compact in size even when the amount of the sheets that can be accommodated in the sheet accommodating device is increased.
A spring constant of the second urging member may be equal to a weight per unit thickness of the stack of sheets on the stacking portion. That is, the second urging member acts to move the stacking portion downward by an amount corresponding to a thickness of the sheets added. As the stacked sheets are removed, the second urging member acts to move the stacking portion upward by the amount corresponding to the thickness of the sheets removed.
Therefore, even when the stacked sheets are added or removed, the stacking portion is vertically moved by an amount corresponding to the thickness of the sheets that have been added or removed. Consequently, the sheets on the stacking portion can be held at a certain position at all times, so that there is little variation in the vertical movement and the sheets can be fed with stability.
Further, the support member can support the stacking portion so that a pressing force acting on one end of the stacking portion by the first urging member becomes constant regardless of the number of the sheets stacked on the stacking portion.
Therefore, even when the thickness or weight of the sheets to be stacked on the stacking portion is changed, the pressing force acting on the one end of the stacking portion becomes nearly constant regardless of the weight of the sheets to be stacked on the stacking portion. That is, the stacking portion always presses the sheets upward with a nearly constant pressing force by the urging force from the first urging member, so that the sheets can be fed with stability.
In particular, the support member supports the stacking portion at a position expressed by X that satisfies an equation below.
Yxe2x88x922XZ=(F=nearly constant)
wherein:
Y is the urging force from the first urging member;
X is the offset from the center of gravity in a back and forth direction of the sheet;
Z is the weight per unit length of the stack of sheets;
F is the pressing force acting on one end of the stacking portion.
Even when the urging force Y from the first urging member is changed in accordance with change of the weight per unit length Z of the sheet caused by changing the number of sheets, the stacking portion is supported by the support member at a position where the pressing force F acting on the one end of the stacking portion becomes nearly constant at all times. Therefore, the sheet stacking portion presses the sheet upward with a constant pressing force by the urging force from the first urging member, so that the sheets can be fed with stability.
In other words, in accordance with the weight change of the sheets to be stacked on the stacking portion, the pressing force acting on the one end of the stacking portion is maintained at nearly constant value by changing a position where the support member supports the sheet staking member as necessary, so that the sheets can be fed with stability regardless of the number of the sheets.
Further, when the variation of the pressing force acting on the one end of the stacking portion is determined xc2x110%, the sheets can be fed more stably.
Furthermore, when the pressing force acting on the one end of the stacking portion is between 100-600 gf, the pressing force acts on the one end of the stacking portion at all times, so that the sheets can be fed with stability.